The present invention relates to a control system for controlling the operation of a steam turbine and, more particularly, to a steam turbine control system which affords a startup of the turbine and a load variation on the turbine in a minimized time, without causing the thermal stress generated in the turbine to exceed a predetermined limit.
As is well known to those skilled in the art, a large thermal stress is caused in the steam turbine, especially at the portion of the rotor confronting the labyrinth packing behind the first stage, when the steam turbine is started up or subjected to a load variation. The larger the rate of change of speed or load becomes, the larger the thermal stress grows. Therefore, from a view point of safe operation of the turbine, a quick startup and an abrupt load change are strictly forbidden.
Meanwhile, there has been proposed and actually carried out a new method of turbine control. According to this method, the startup and the load change of the turbine is made at a rate as large as possible but would never cause a thermal stress exceeding a predetermined limit which has been drawn for each of repeated startup and load change from a view point of observation of the life consumption rate of the turbine. A practical example of this method is proposed, for example, in the specification of U.S. Pat. No. 3,588,265 entitled "System and method for providing steam turbine operation with improved dynamics". Although quite effective in achieving the above stated purpose, unfortunately, this newly proposed method is applicable only to such turbines as having an impulse chamber, because it relies upon a measurement of the temperature in the impulse chamber as a parameter of the turbine control. Thus, this newly proposed method cannot be directly applied to the control of turbines having no impulse chamber. In this newly proposed method, the temperature in the impulse chamber is measured as the parameter or representative of the temperature at the point downstream or behind the first stage, at which the thermal stress is most severe and, therefore, has to be observed strictly.
Thus, for optimumly controlling the steam turbine having no impulse chamber, it is necessary to take one of the alternative measures of measuring directly the steam condition at the point behind the first stage or estimating that condition from the data available at the outside of the turbine. The first-mentioned direct measurement is, however, practically impossible to carry out. Thus, the turbine control is obliged to rely upon the second-mentioned measure, i.e. an estimation.
In the turbine control relying upon this estimation, the following requisites are indispensable.
Firstly, it is essential to make a calculation of the thermal stress at a high precision. This high precision of calculation of thermal stress is required in all conditions of turbine operation including no-load running, load running, putting into synchronous parallel running and so on.
Secondly, the turbine control must be able to startup the turbine safely and without fail. To this end, the steam regulating valve at the turbine steam inlet has to be controlled upon confirmation of not only the instant thermal stress but also the future thermal stress not exceeding the previously drawn limit, because the thermal stress actually appears with certain time lag behind the change of the steaming condition of the turbine. At the same time, the turbine condition has to be relaxed to the safe region without delay, if a thermal stress exceeding the limit or other extraordinary condition is experienced or expected.
Thirdly, the arithmetic or calculation for the estimation of thermal stress and other purposes has to be made by means of digital signals, without necessitating uneconomically large computer. Further, the turbine control system must perform the turbine control at a suitable time interval.
Other improved turbine control systems have been proposed in, for example, in the specification of U.S. Pat. No. 3,446,224 entitled "Rotor Stress Controlled Startup System" and in the specification of U.S. Pat. No. 3,959,635 entitled "System and Method for Operating a Steam Turbine with Digital Computer Control Having Improved Automatic Startup Control Features". However, these improved systems suffer, more or less, the above stated problems of the prior art.